How TOML dates become strings
JSON has no date type. ConfigKit writes the parsed TOML date as text and keeps the offset you wrote.
The example
An offset datetime stays an offset datetime, written as a string. Milliseconds are added because the parsed time has millisecond precision.
offset = 1979-05-27T07:32:00-08:00
The JSON string is 1979-05-27T07:32:00.000-08:00. A UTC time such as 2024-01-15T10:30:00Z becomes 2024-01-15T10:30:00.000Z.
The edge case
A local date has no time and no offset, so it stays a calendar date. A local time stays a clock time. Neither is shifted to UTC.
local_date = 2024-01-15 local_time = 10:30:00
Those become 2024-01-15 and 10:30:00.000. The same strings are what TOML to YAML writes.
Try it on TOML to JSON or TOML to YAML.